1. Field of the Invention
The present invention relates to an optical information recording medium, and particularly to an optical information recording medium rewritable in a heat mode.
2. Description of the Related Art
With the beginning of BS (Broadcast Satellite) digital broadcasting in high definition television (HDTV) image quality just around the corner, the development of large capacity recording media, capable of recording BS digital broadcasting in HDTV image quality for about 2 hours, is advancing. In an optical disc for a violet laser beam source, regarded as having high potential as the next generation of optical disc, a recording layer is formed on a substrate by coating a thin cover layer having a thickness of 0.1 mm to 0.3 mm. In a system using the optical disc, the laser beam is throttled using an objective lens with a high numerical aperture (hereinafter referred simply as to NA) of 0.65 or higher for a pickup, and recording is carried out by irradiation with a laser beam from a thin cover layer side of the optical disc to achieve a one-sided recording capacity as high as 22.5 GB.
Also, with respect to ISOM 2000 (International Symposium on Optical Memory 2000), DVR-Blue, which is a phase change medium using a violet laser, has been released.
However, in developing such an optical disc for use with a violet laser light source, one method employed for increasing a density of the optical recording medium has been to shorten the wavelength of the laser to be employed and to increase the NA of the objective lens. In such a case, a laser spot becomes small and a distance between the pickup and the medium (a working distance) is narrowed, so that a problem occurs in that a roughness of the medium surface, which scarcely has any effect in the case of a conventional optical recording medium, does have effects on this medium.
In other words, if the slight unevenness of the recording medium surface is significant, diffused reflection of light from the medium surface occurs, resulting in problems such as decreased reflectivity, deterioration in the shape of marks to be recorded, and lowered C/N and jitter. Over a wider area, it is observed that a focus margin and a tilt margin are narrowed when the roughness (waviness) is great, and further that the disc and the pickup, which generally do not make contact, are brought into momentary contact with each other due to surface movement and tremor of the disc. Accordingly, the disc surface is easily scratched causing errors at the time of reproduction (playback).
As described above, a high density optical information recording medium (DVD), capable of recording and reproducing a large quantity of text information, image information, and audio information, is increasingly in demand, and, in particular, research of even greater density recording using an optical information recording medium is being carried out in order to record digital high vision TV broadcasting. As a method for performing high density optical recording, the wavelength of a laser beam employed for recording and reproducing with the optical information recording medium has been further shortened and the numerical aperture (NA) of an objective lens employed in an optical system of a recording and reproducing apparatus has been further widened.
A previously known phase change type optical disc, comprises an alloy layer, such as GeSbTe, as a recording layer, and recording and reproducing are carried out by irradiating the recording layer with a laser beam to instantaneously heat the irradiated portion and change a state thereof from a crystalline state to an amorphous state and by utilizing the changed reflectivity resulting from the phase change. Recently, the DVR-Blue system, in which recording and reproducing are carried out by a violet laser using a recording layer of the phase change type, was disclosed (ISOM 2000, pp. 210–211).
This system employs a land-groove recording method, which refers to a method for carrying out recording at both groove portions of a guiding groove for a laser beam and land portions (hill portions) formed on a substrate and which is a recording method employed for DVR-RAMs (rewritable type). On the other hand, DVDs include those of DVD-RW (rewritable type) and DVD+RW (rewritable type) standards, and a groove recording method for carrying out recording only in the groove is employed.
As compared with the groove recording method, the land-groove recording method is suitable for achieving high density since a track pitch is easily increased. The recording tracks, however, are so close to one another that problems such as cross erasing and cross stroking affecting tracks already recorded by the laser are apt to take place when recording is carried out in tracks adjacent to the already recorded tracks. Furthermore, conversion to DVD-ROM is difficult.
In order to solve such problems, a phase change medium for DVR-Blue, in which a method of recording only in grooves is employed, has been disclosed [ODS (Optical Data Storage) 2001, pp. 139–141].
Further, among DVDs, a writable DVD-R, capable of undergoing recording once, is known. Since a dye is applied for recording on the DVD-R, the DVD-R utilizes a groove recording method allowing easy dye accumulation and having balanced properties.
Generally, when the wavelength of the laser beam to be employed is shortened and the NA of the objective lens is increased, the common energy of the light increases in inverse proportion to the wavelength. Accordingly, when the wavelength of the laser beam is shortened in order to increase the density, the light energy radiated to the medium is increased. Therefore, durability with respect to repeat reproduction becomes an issue. In order to assure reproduction durability, a method of lowering the laser output at the time of reproduction can be used. When the reproduction output is lowered, however, the reflectivity is decreased, resulting in RF output decrease, C/N decrease, and jitter deterioration.
A DVD-R is manufactured by applying a dye to a substrate A to form a light reflective layer, subsequently forming a protective layer, and adhering the resulting substrate to another substrate B. At the time of recording and reproducing, a laser beam is radiated to groove portions (convex portions, when observed from a substrate A side of the medium) from the substrate A side. Generally, the dye is thicker in the groove portions, and, from the standpoint of shape, heat accumulates more easily in the groove portions at the time of laser beam irradiation. As a result, the groove portions are superior to land portions (concave portions, when observed from the substrate A side of the medium) in modulation degree, C/N, and jitter.
In the DVR-Blue system, a light reflective layer is formed only on a substrate A, a dye is further applied thereto, and a cover layer is formed thereon. Recording and reproducing are carried out by radiating a laser beam from a cover layer side, and, in contrast to a conventional DVD, the groove portions are concave portions when observed from the cover layer side. Such a structure, therefore, leads to problems including RF output decrease, C/N decrease, and jitter deterioration.